Over the years thousands of people have been convinced that we Earthlings have been visited by aliens from other civilisations.
But there's one problem with this - the number of aliens actually discovered is zero.
There are lots of photos and films of apparent sightings but most have a logical explanation like aeroplanes and military exercises.
Or they're hoaxes!
But others say comets could have helped start life on Earth. They're not certain but if they're right it means that we could all be aliens!